A prevalent class of two-dimensional electro-optical display devices, for displaying digitally provided images, is a flat transmissive device wherein the transmission at each pixel is controllably modulatable. The most common type of this class is the Liquid-Crystal Display (LCD), which is widely used in computers, especially of the portable kind, and in small display systems for viewing through ocular optics. Every such display device must be provided with a light source that illuminates its back face so that its light is transmitted across the device, while being image-wise modulated by it. This light source is commonly referred to as a “backlight”. The primary requirement for such a backlight is that its brightness, as viewed from the front, is of a sufficiently high overall level, and that it is relatively uniform over the face of the display. In the case of computer displays, there is another major requirement, namely that the backlight be relatively thin—in keeping with the thinness of the display device itself and thus forming a complete display assembly whose depth dimension is appreciably smaller than any dimension of its face. In what follows any light transmissive display device will be referred to as a LCD, without detracting from the applicability of the invention to other types of transmissive display devices.
For large display devices, backlights have traditionally been constructed of an array of fluorescent tubes behind a light diffusing screen. Such traditional backlights are characterized by poor brightness uniformity and by small aspect ratio. Aspect ratio (AR) is defined as the ratio of the diagonal size of the illumination window (or illumination aperture, as it will be termed hereafter) to the maximum depth dimension of the backlight.
A more recent type of backlight—the so-called edge coupled type—employs a light source coupled to an edge of a light guiding plate (LGP), along which the light flux propagates by total internal reflection (TIR) with almost no losses. This enables constructing backlights with very large AR—typically of 50-100—for 10-20″ diagonal LCDs used in existing portable and desktop computers. In these types of devices a light source, usually a cold cathode fluorescent lamp, introduces light into a light guiding plate (LGP), through an edge surface thereof. The LGP is so structured that part of the light entering through the edge radiates out through the LGP's front face. The LGP is so oriented that its front face or surface is parallel to the faces of the display device and proximate thereto, and thus light radiating from the LGP illuminates the display device and is transmitted there through. If the display device is a LCD, there is also typically disposed between it and the LGP a polarizing sheet. Several additional components are often used to increase the brightness and uniformity of the transmitted light, these include:                a reflector behind the lamp;        reflectors behind the LGP's back face and at other edge surfaces;        one or two orthogonal oriented lenticular films in front of the LGP's front face;        a diffusing film behind the display device.        
Small-aperture LCD displays, those with a display area (and therefore also illumination aperture) of 0.5″ to 2.0 inches in diagonal size are commonly used for small devices such as pagers, cellular phones, digital cameras, camcorders, personal digital assistants, and especially for small head mounted VGA and SVGA displays for virtual reality systems. The ubiquitous technology for small-aperture LCD displays is the so-called flat fluorescent backlights. These are typically about 10 mm thick, weigh about 20 g, have an aperture ratio of 4:3, have uniformity (ratio of highest to lowest brightness in the aperture) of 1.6:1, and produce a surface luminance of approximately 1200 fL.
Due to its inherent compactness, ease of operation and luminance efficiency, a much more suitable type of light source for such applications (instead of fluorescent lamps) is a light-emitting diode (LED). FIG. 1A shows a prior art cavity type LED-based backlight 10 that is analogous to an integrating sphere as described, for example, in U.S. Pat. Nos. 5,892,325 and 6,134,092. The interior of the cavity 12 is typically constructed of a white diffusely reflecting material of high reflectivity (greater than 95-99%) and backed with a surface of a diffusive, reflecting material 14. The LED light source 14 is coupled to an edge of the cavity 12 and along with the cavity is disposed in a housing 18. Disposed adjacent and proximate to the front surface of the cavity, e.g., the surface opposite the surface backed with the diffusive material, are the one or more layers 20a, 20b, and 20c of brightness enhancing films (BEFs), diffusers and materials described below and unique to the different types of LCDs. Finally disposed in front of the layers 20a-20c is the image display device, namely the LCD 22
Another prior art backlight using multiple LEDs coupled to two edges of a planar LGP is depicted in FIG. 1B. The use of multiple LEDs is dictated by a need to improve the luminance uniformity, which is poor (remains below an acceptable value) with one LED in existing devices. With recently developed high flux, “Ultra-Bright” LEDs a smaller number of more efficient LEDs can be used to produce a given display luminance. A prerequisite for such a device is a maximally thin LGP-based optical architecture accomplishing significantly better luminance uniformity, which is an object of the present invention. Multiple LEDs can also be used to attain higher display luminance as is needed in high information density (high resolution) graphic LCDs for 3G wireless devices, PALMS etc.
While more compact than the previously mentioned devices, the device of FIG. 1A still suffers from several practical problems. First, the device of FIG. 1A is still very large in all dimensions with respect to the size of the illumination aperture and has the additional disadvantage of being energetically inefficient. In practice, the requirement of uniform illumination with cavital design is difficult to achieve, in particular for larger and thinner configurations, i.e., larger aspect ratio designs. Indeed, the performance of such a device imposes two conflicting requirements on AR. On one hand, to achieve brightness uniformity the AR should be small, assuring that all of the cavity's surfaces projecting through the exit aperture are uniformly illuminated by a relatively small number of multiple interreflections (MIRs) of the highly nonuniform light flux emanating from the light source. This reduces reflection losses and increases efficiency. On the other hand, the AR should be sufficiently large to allow a commercially and technically acceptable depth dimension of a device. As a result, prior art devices are bulky in terms of depth dimension and/or cannot assure an acceptable uniformity for larger displays. Typical devices require that 4<AR<10 as specified in, for example, U.S. Pat. No. 6,043,591, while analysis indicates that the achievable uniformity with such designs is only on the order of 1.3:1. While these performance criteria may be suitable for some LCD applications, they are not sufficient for applications using multi-colored LED sources, as described below, in which a uniformity between the illumination colors of 1.05:1 is required in order to prevent perceivable color shifts.
A partial solution known as a compound cavity-TIR system is described in U.S. Pat. No. 6,043,591, which suggests filling the cavity with a fluid. However, the internal reflections within the fill medium are still diffuse at all lambertian cavital surfaces, as a result of direct optical contact. In fact, only an upper surface of such filled cavity acts as a light guide, but its ratio to the total surface area of the cavity is too small to have any significant positive effect. As a result, such systems fail to appreciably improve the backlight luminance efficiency and achievable AR.
As noted above, the best solution to date for small-aperture LED-illuminated backlights would seem to be the use of a light guiding plate (LGP), which distributes the light flux by total internal reflection (TIR). However as realized herein, several practical problems inherent to such a system are compounded in the case of small-aperture LGP-based backlights. A first of these practical problems include the fact that a relatively thick planar LGP, with 1 <AR<10, suffers from elevated light flux losses, since, with existing extraction means, a large fraction of the LED-injected flux inevitably reaches the opposite edge of the LGP and is coupled out on the proximal outer reflector; it is then coupled again into the LGP, travels in a reverse direction and eventually ends up on the LED, where it is totally or partially absorbed. An optimal LGP should reduce this residual flux and ensure that a maximal fraction of the initially forward propagating flux from the LED should be extracted in a first pass. Even with thinner LGPs, e.g., 1-2 mm, having optimized extractor distributions described below, the AR is still relatively small so as to make this problem significant.
Light extraction in a flat illumination device (“FID”) can also be effected by using a tapered LGP the faces of which are mutually inclined at some angle THETA, thus forming a wedge, rather than a planar parallel-faced plate. As the flux from the lamp, coupled to the edge of the plate, propagates along the wedge, the angles of incidence ΠHI are reduced by 2θ at each reflection from the inclined face(s). When the angle φ of any flux component becomes smaller than a critical angle of TIR, this flux is coupled out from both faces of the LGP in a number of successive reflections in a forward direction, following the Fresnel equations. The flux is extracted at the directions close to the grazing angles and some internal or external diffusing and/or concentrating elements can be used to modify the spatial luminance, or luminous intensity distribution, to satisfy the particular requirements. Some of the forward propagating flux reaching the opposite and adjacent LGP edges is eventually reflected by an external reflector into a backward path, as well as sideways skewed paths. Linear one dimensional (i.e., having constant inclination angle along one orthogonal direction) wedge-like LGPs are described in relation to a number of FIDs, in, for example, U.S. Pat. Nos. 6,104,455 and 6,259,496. However, these linear wedge shaped devices inherently produce a significantly non-uniform luminance, this non-uniformity growing with the LGP's length.
U.S. Pat. No. 5,357,405 describes a nonlinear semicylindrical concave surface which effects, in combination with additional light extracting means, better uniformity. This one-dimensional nonlinear wedge is not designed to produce uniform luminance independently. U.S. Pat. Nos. 5,303,322; 6,002,829 and 6,044,196 describe the possibility of using a one-dimensional nonlinear wedge for compensating light output irregularities for a special type of tapered multilayer devices that are very different from the FIDs under consideration. These patents fail to teach any practical solutions and in fact, the expanding convex wedges qualitatively depicted therein necessarily suffer from augmented non-uniformity as compared to linear wedges. Further, the attempt to analyze the problem using general adiabatic invariant cannot produce any meaningful solution since one has to consider the exact convolutions of Fresnel equations in three dimensional domain with complex boundary conditions, imposed by the LGP shape, and backward propagating residual flux.
Apart from reducing average thickness and bulk material, the wedge can be used for effective light extraction in the first pass and reduction of the residual flux and accompanying losses. Indeed, it directly follows from the General Photometric Invariant (so-called etendue conservation principle), that a total flux extracted from horisontal face(s) of a one dimensional wedge in a first forward pass is proportional to:(Zmax −Zmin)/Zmax
where Zmax, and Zmin—are respectively maximal and minimal thickness of the wedge.
A second practical problem resides in the fact that in order to attain high illumination uniformity, light extraction from the LGP's internal flux should be nonuniform. In relatively large backlights, illuminated by extended tubular lamps (such as cold cathode fluorescent lamps), the extractors' density distribution over the face of the LGP should be greater the further away they are from the source. This is illustrated in, for example, U.S. Pat. Nos. 5,283,673; 5,796,450; 5,949,505; and 5,896,119. Indeed, flux density inside the LGP having some extractor means is generally not uniform and diminishes gradually with increasing distance from the light source. Thus, if the extractors were to be uniformly distributed over the face, extracted light intensity would likewise vary across it. In order to overcome this phenomenon, the extractor elements in prior art devices are distributed non-uniformly, being more sparse near the lamp and more crowded near the opposite edge. Light extractor areas are characterized by a cover factor (CF), representing a ratio of extractor area to an elementary unit area, located anywhere within a light extracting face(s) of an LGP.
A third practical problem related to the fact that an efficient coupling architecture is required to take the light emitted from a LED source and inject it efficiently into a thin lightguide. LEDs with conventional primary optics (lens-like or flat shaped epoxy encapsulants in direct optical contact with the LED emitter and/or reflector cup) suffer from very significant losses due to Fresnel retroreflection of initially emitted flux. This phenomenon takes place during the passage of radiation at the interface between the LED emitter (N=3.5−3.7) and the encapsulant (N=1.5−1.6) and at the interface between the encapsulant and air. Similar losses also occur for LEDs with a cup-shaped or cup reflector surrounding the emitter. In the latter case some of the flux reflected by such a reflector reaches an emitter or strikes an encapsulant-air interface at large angles causing an augmented retroreflection. Most of the thus retroreflected flux is absorbed in the LED, causing output losses and eventual elevation of LED chip temperature, which reduces the LED's luminous efficacy. This is a problem as current LED backlights only have optical efficiencies in the range of 50-75% and uniformity typically in the 1.3-1.4:1 range.
The discussion so far has not included the subject of color. This subject is important even for monochromatic displays. In fact, because of their inherent spectral characteristics, the use of LEDs makes white illumination problematic, but at the same time may also provide advantages when applied according to the present invention, all as described herein below.
A typical “white” LED, made by Nichia, Ltd. consists of a bright blue LED covered with a yellowish phosphor coating. This backlight has approximately the same dimensions as the flat fluorescent type (described above), weighs about 8 g, has a uniformity of about 1.4:1 and emits 1500 fL. Any backlight that uses a white lamp as the light source, including the above-mentioned white LED, has an important drawback, namely that the spectrum of the emitted light is fixed and is determined almost solely by that of the lamp. This, in turn, determines the absolute color of the display, if monochromatic, and of white portions (and consequently also of other portions) of a displayed color image, which color is also known as the “white point”. In many applications, whether for monochrome or for color displays, it is important to be able to control the white point. In the aforementioned conventional type of backlights such a control is very difficult, in that it can only be accomplished by carefully selecting the lamp or by interposing suitable correction filters. Moreover, the white point may change with the life of the lamp.
To display color images, the common practice is to employ a transmissive display device, such as an LCD, in one of several different arrangements. A first arrangement includes an array of color filters, usually of the three additive primary colors (red, green and blue), congruent to a suitable array of light modulating elements, or pixels. In this first arrangement, known as a filter-array arrangement, A backlight, such as described hereabove, is employed for such a color LCD in much the same manner, as long as the spectrum of its emitted light is broad enough to include the transmission spectra of all the filters. In operation, the light transmitted through any modulation element of the LCD, is spectrally filtered by a corresponding filter; all elements corresponding to red filters thus form the red component image and the green and blue components are similarly formed. Because of the small size of the elements, relative to the resolution of a human eye, the three images combine in the observer's eyes into a continuous full-color image.
This practice has several major drawbacks. First, appreciably less than one third of the light energy emitted by the backlight is transmitted by each filter and thus the apparent brightness of the display, even in white areas of the image is considerably lower than it would have been with a monochromatic LCD device, given the same lamp intensity. In other words, the display efficiency is considerably reduced. A second drawback is that color-filter-array type LCDs has relatively high cost of manufacturing due to the intricacies of the manufacturing process. A third drawback relates to the fact that, for a given pixel resolution, the basic LCD resolution must be at least three times higher (per unit area). This last drawback has become a particular liability in the case of small-aperture display devices, especially as they simultaneously strive for higher resolution, which correspondingly puts a premium on pixel real estate, while requiring even more pixels in the shrinking space.
A second arrangement, known as Color Field Sequential Imaging (CFSI) method for transmissive color displays is also known in the art. This second arrangement basically consists of a monochrome LCD and three light sources, each of a respective primary color, illuminating its back. Signals corresponding to the three primary-color component images are applied to the LCD sequentially, in a regular cycle. Synchronously with the application of each such component a corresponding one of the three light sources is switched on so as to illuminate the LCD while it image-wise modulates the transmitted light according to the corresponding color component. All three color components are thus sequentially displayed for each frame of video and therefore their rate is three times the regular video rate (e.g. 180 Hz). Because of the image retention characteristics of the human eye, all three components are effectively merged into a single full-color image corresponding to the respective video frame.
A sequential color display type, such as described herein, inherently overcomes the three drawbacks of filter-array display type devices as they allow practically all the light energy that is applied to the LCD over white areas of the image to be transmitted. Further, the LCD itself is a monochromatic type and thus relatively inexpensive both in terms of components and in terms of the manufacturing process. Finally, the relative intensities of the three light sources may be adjusted so as to achieve any desirable white point.
In order to illuminate the above-mentioned sequential color LCD, a backlight with the ability to iterate quickly enough between the three basic colors is needed. This is provided by a LED-based backlight architecture, using very bright red, green and blue (RGB) LED's to create uniform fields of sequential RGB light. Since LED's can switch on and off in 15 nanoseconds, they can succeed in this application, whereas RGB fluorescent lights cannot because of the long fluorescent decay times between successive on/off states of the RGB phosphors. LEDs also inherently possess the desirable characteristics of maximum color saturation and high photonic efficiency. According to prior art, LEDs cannot, however, be practically used to illuminate the edge of a LGP to serve as a backlight, because each is, in effect, a point source of light (as opposed to the elongated format of the light emitted by CCFLs), which causes the resulting pattern of light flux emitted from the face of a typical LGP to be highly non-uniform. Therefore, in most prior art backlighting devices, LEDs are positioned in back of the LCD, and not coupled along an edge.
A typical prior-art arrangement, with three LEDs positioned in the back of a diffusing/redirecting screen 30, and enclosed in a housing 34, is shown schematically in FIG. 1C. The LEDs 32 are at a considerable distance from the screen 36, in order to minimize the non-uniformity of illumination over the screen due to the varying distances from the sources. The device 30 typically will further include one or more films or lenses for conditioning the light emitted from the LED. In this case, a Brightness Enhancing Film (BEF) 36 and a diffuser 38 show a typical arrangement. Such prior-art type of colored backlight has a major disadvantage of having a very large depth dimension, contributing to bulkiness of the entire display device. Further disadvantages of such prior-art devices are that the LEDs themselves have a non-uniform radiation pattern, which further contributes to the non-uniformity of the backlight, and that the three LED sources must still be placed at some mutual distances, which causes non-uniformity in the hue of white over the display, as discussed above. U.S. Pat. No. 5,892,325 to Gleckman discloses a backlight comprising a diffusive reflective cavity, which is illuminated from its side, in one configuration—by a plurality of red, green and blue LEDs. This device, however, suffers from the disadvantages already discussed above.
Thus, there exists a need for a backlight for LCDs that provides the advantages of colored LEDs while addressing the limitations inherent is such arrangements. Such a device must be able to monochromatically or color-sequentially illuminate a monochromatic LCD, and have attributes including:                                    Uniform luminance over the entire illumination aperture;            Uniform color over the entire illumination aperture;            High brightness efficiency in utilizing a given light source; and            Compact overall dimensions and a thin structure.Preferably the backlight should use LEDs and light-guiding components and also be inexpensive to manufacture.                        